Ariel (Atmospheric Remote-sensing Infrared Exoplanet Large-survey) is an ESA M class mission aimed at the study of exoplanets. The satellite will orbit in the lagrangian point L2 and will survey a sample of 1000 exoplanets simultaneously in visible and infrared wavelengths. The challenging scientific goal of Ariel implies unprecedented engineering efforts to satisfy the severe requirements coming from the science in terms of accuracy. The most important specification – an all-Aluminum telescope – requires very accurate design of the primary mirror (M1), a novel, off-set paraboloid honeycomb mirror with ribs, edge, and reflective surface. To validate such a mirror, some tests were carried out on a prototype – namely Pathfinder Telescope Mirror (PTM) – built specifically for this purpose. These tests, carried out at the Centre Spatial de Liège in Belgium – revealed an unexpected deformation of the reflecting surface exceeding a peek-to-valley of 1µm. Consequently, the test had to be re-run, to identify systematic errors and correct the setting for future tests on the final prototype M1. To avoid the very expensive procedure of developing a new prototype and testing it both at room and cryogenic temperatures, it was decided to carry out some numerical simulations. These analyses allowed first to recognize and understand the reasoning behind the faults occurred during the testing phase, and later to apply the obtained knowledge to a new M1 design to set a defined guideline for future testing campaigns.
Ariel (Atmospheric Remote-Sensing Infrared Exoplanet Large Survey) is the adopted M4 mission in the framework of the ESA “Cosmic Vision” program. Its purpose is to conduct a survey of the atmospheres of known exoplanets through transit spectroscopy. Launch is scheduled for 2029. Ariel scientific payload consists of an off-axis, unobscured Cassegrain telescope feeding a set of photometers and spectrometers in the waveband between 0.5 and 7.8 µm and operating at cryogenic temperatures (55 K). The Telescope Assembly is based on an innovative fully-aluminum design to tolerate thermal variations avoiding impacts on the optical performance; it consists of a primary parabolic mirror with an elliptical aperture of 1.1 m of major axis, followed by a hyperbolic secondary that is mounted on a refocusing system, a parabolic re-collimating tertiary and a flat folding mirror directing the output beam parallel to the optical bench. An innovative mounting system based on 3 flexure-hinges supports the primary mirror on one side of the optical bench. The instrument bay on the other side of the optical bench houses the Ariel IR Spectrometer (AIRS) and the Fine Guidance System / NIR Spectrometer (FGS/NIRSpec). The Telescope Assembly is in phase B2 towards the Preliminary Design Review to start the fabrication of the structural model; some components, i.e., the primary mirror, its mounting system and the refocusing mechanism, are undergoing further development activities to increase their readiness level. This paper describes the design and development of the ARIEL Telescope Assembly.
AIRS is the infrared spectroscopic instrument of ARIEL: Atmospheric Remote‐sensing Infrared Exoplanet Large‐survey mission selected in March 2018 as the Cosmic Vision M4 ESA mission and planned to be launched in 2029 by an Ariane 6 from Kourou toward a large amplitude orbit around L2 for a 4 year mission. Within the scientific payload, AIRS will perform transit spectroscopy of over a 1000 of exoplanets to complete a statistical survey, including gas giants, Neptunes, super-Earths and Earth-size planets around a wide range of host stars. All these collected spectroscopic data will be a major asset to answer the key scientific questions addressed by this mission: what are the exoplanets made of? How do planets and planetary system form? How do planets and their atmospheres evolve over time? The AIRS instrument is based on two independent channels covering the CH0 [1.95-3.90] µm and the CH1 [3.90-7.80] µm wavelength range with prism-based dispersive elements producing spectrum of low resolutions R<100 in CH0 and R<30 in CH1 on two independent detectors. The spectrometer is designed to provide spectrum Nyquist-sampled in both spatial and spectral directions to limit the sensitivity of measurements to the jitter noise and intra pixels pattern during the long (10 hours) transit spectroscopy exposures. A full instrument overview will be presented covering the thermal mechanical design of the instrument functioning in a 60 K cold environment, up to the detection and acquisition chain of both channels based on 2 HgCdTe detectors actively cooled down below 42 K. This overview will present updated information of phase B2 studies in particular with the early manufacturing of prototype for key elements like the optics, focal-plane assembly and read-out electronics as well as the results of testing of the IR detectors up to 8.0 μm cut-off.
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